docs: def overlapping_points(coords_a, coords_b, cutoff, periodic=False):
      '''Return the indices of *coords_b* points that overlap with
      *coords_a* points. The overlap is calculated based on *cutoff*.
  
      **Parameters**
      
      coords_a: np.ndarray((NA, 3))
      
      coords_b: np.ndarray((NB, 3))
      
      cutoff: float
         Distance within two points are considered overlapping.
      
      periodic: False or np.ndarray(3)
         Periodicity in x, y, z dimensions
      
      '''
      
      res = distance_matrix(coords_a, coords_b, periodic=periodic, 
                            cutoff=cutoff, method="cell-lists")
      overlaps = res.nonzero()[1]
      return np.unique(overlaps)